The present invention relates to an optical scanning device having a plurality of light sources, an optical scanning method, a pixel forming device, an image forming apparatus using the optical scanning device having a plurality of light sources, and a color image forming apparatus using the same.
FIG. 29 illustrates an example of the structure of an image forming apparatus of a general type, such as a laser printer utilizing an electro-photography process, a digital copier, and so on. Referring to FIG. 29, laser beams emitted from a semiconductor laser unit 1001 acting as a light source unit are deflection-scanned with a rotating polygon mirror 1002 and irradiated onto a photosensitive member 1004 to be scanned through a scanning lens (fθ lens) 1003. The photosensitive member 1004 is exposed the laser beams and an electrostatic latent image is formed on the surface of the photosensitive member 1004. A phase synchronizing circuit 1009 forms an image clock (pixel clock) synchronizing with an output signal of a photodetector 1005 detecting the position of a scanning line and sends the image clock to an image processing unit 1006 and a laser driving circuit 1007. The image processing unit 1006 and the laser driving circuit 1007 are controlled in synchronization with the image clock, so as to control a light emitting time of the semiconductor laser beams. This control allows formation of the electrostatic latent image of the image data formed with the image processing unit 1006 at an appropriate position on the surface of the photosensitive member 1004.
Recently, there have been increasing demands for speeding-up a printing speed (i.e., forming an image at a high speed) and enhancing an image quality. In order to meet such demands, the speeding-up of the polygon motor working as a deflector and the image clock working as a reference clock for a laser modulation have been achieved. The efforts for speeding-up both of these means, however, have been approaching to the limits, and a further speeding-up of these means is being confronted with a severe situation.
Therefore, the speeding-up has been attempted by adopting multi-beams using plural light sources. An optical scanning method using multi-beams can decrease a rotating speed of the polygon motor working as the deflector and frequency of the image clock due to an increased number of light fluxes capable of being scanned simultaneously with the deflector, thereby achieving a high-speed and stable optical scanning and image forming.
As the light sources for emitting such multi-beams, there may be used a combination of plural laser chips of single beams, an LD array with plural light emitting diodes incorporated into a laser chip, and so on.
Since a semiconductor laser to be used for the LD array or the like is extremely compact in size and can be modulated directly with driving electric current at a high speed, it has recently been used widely as a light source for laser printers and so on. As the driving electric current and light output of the semiconductor laser have the characteristic of being varied with temperature, however, the semiconductor laser has the problem that the light output be varied with the heat emitted from the semiconductor laser itself. In particular, a vertical-cavity surface-emitting laser with plural light sources disposed on an identical chip is likely to be affected by a variation in temperature or temperature crosstalk, etc., by light emission or extinction because a distance between the light sources is short. This may likely cause changing a quantity of light.
In order to compete with this problem, for example, Japanese Patent Publication No. 2001-272615 discloses an optical scanning device with plural light sources disposed in a two-dimensional way for optically scanning a medium by deflecting plural light fluxes, in which a density of disposition of light emitting points is made maximum without causing an occurrence of any crosstalk by heat generation among the light emitting points.
Japanese Patent Publication No. 2003-72135 discloses an image forming apparatus using a vertical-cavity surface-emitting laser, which is provided with a driving part capable of varying a light emitting intensity of each chip at a pixel unit and a control part for controlling a light emitting time and forms an electrostatic latent image of pixels while avoiding a deterioration in the light emitting properties of the laser array caused by heat generation by controlling the light emitting time.
Japanese Patent Publication No. 2001-350111 discloses a multi-beam scanning device using a vertical-cavity surface-emitting laser, which can avoid the problem with crosstalk caused by heat generation among light emitting points by way of a defined arrangement of the light sources and realize the recording of images at a high density.
In general, conventional optical scanning devices having plural light sources as described above have the problem that a deviation in a light emitting level of each light source directly affects an adverse influence on a variation in brightness (or concentration) of pixels because one pixel is drawn by one light source. In particular, they have the problem that such a variation in brightness (or concentration) of pixels located in a sub-scanning direction which are discontinuous in terms of time cannot be corrected by such conventional systems. Further, they have the problem that there is no effective way for highly accurately correcting a deviation in position of the scanning line in the sub-scanning direction, which may be caused by optical or mechanical factors.